Method of producing a corrosion resistant internal surface on a container made of light metal



Oct. 30, 1962 P. BRENNER ETAL 3,061,447

METHOD OF PRODUCING A CORROSION RESISTANT INTERNAL SURFACE ON A CONTAINER MADE OF LIGHT METAL Filed May 22, 1959 2 Sheets-Sheet 1 Jnvenfoa:

M MSM Wam Oct. 30, 1962 P BRENNER ETAL 3,061,447

METHOD OF PRODUCING A CORROSION RESISTANT INTERNAL SURFACE ON A CONTAINER MADE OF LIGHT METAL Filed May 22, 1959 2 Sheets-Sheet 2 Fig. 5 8

Jnvenfors": W

United States Patent 3,061,447 METHOD OF PRODUCING A CORROSION RESIST- ANT INTERNAL SURFACE ON A CONTAINER MADE OF LIGHT METAL Paul Brenner, Bad Godesberg, Franz Kraus, .Koln-Lmdenthal, and Josef Sehulte, Bonn, Germany, asslgnors to Wilhelm Schmidding, Koln-Niehl, Germany Filed May 22, 1959, Ser. No. 815,139 Claims priority, application Germany May 24, 1958 3 Claims. (Cl. 99-181) This invention relates to light metal containers for liquids, and to a method of producing a corrosion-resistant surface on the inside of containers of light metal, i.e. of aluminum or aluminum alloys, especially of portable containers, storage tanks, and the like, which are completely closed except for the provision of openings for filling and/ or emptying them. The invention is primarily concerned with containers made of a base material consisting of a light metal alloy which is capable of sustaining high mechanical stress and which is provided with an internal plating of pure aluminum.

To prevent corrosion it is the practice to provide the inside surfaces of such containers with an oxide film produced chemically or by anodising, such a film being formed by oxidation of the aluminum to the desired depth, so that its presence can be detected in a section through the container wall. Generally, the inner surface of the container is first pickled, for instance with a caustic, before it is oxidised.

However, it has been found that corrosion occurs despite the presence of the oxide film, usually after periods of use of varying lengths, indicating that the cause of such corrosion cannot be ascribed exclusively to the effect of Wear. The reason for the occurrence of such corrosion is in fact to be sought in the conformation of the surface of the material, and this is confirmed by practical experience which shows that even fairly new containers sometimes corrode although they have been plated with pure aluminum. It has, in fact, been hitherto impossible to improve the inside surface of a light metal container in such a way that it is entirely corrosion-proof.

The described phenomenon is observed especially in beer barrels which have been internally plated with pure aluminum. In many instances corrosion sets in after a comparatively short period of use, and it is likely that the cleaning agents used in breweries may be partly to blame. However, the underlying cause of the trouble is the conformation of the surface of the material, and all attempts to treat the internal surfaces of light metal beer barrels to make them corrosion-resisant have so far failed to produce the desired result.

The invention is based on the fact that the internal surfaces of light metal containers exhibit scratches and scores due to the mechanical processing, such as rolling and drawing, which they have experienced, and these are fundamentally capable of making the surface susceptible to corrosive attack. Moreover, the invention takes into consideration the fact that the pickling treatment performed for the purpose of cleaning the surface is itself a deliberately aggressive corrosive process which gives rise to microscopically fine surface pitting. These points on the surface which cannot be subsequently removed by mechanical or pickling treatments cause the grown oxide film to form fissures around these sharp pointed edges because the oxide film fails completely to cover the scratches, scores, or other pointed and sharp edges formed on the surface.

The present invention overcomes these drawbacks by providing in one of its aspects a method of producing a corrosion-resistant internal surface on a container made of light metal, wherein the internal surface of the conice tainer is subjected to a lustring treatment to flatten it and to smooth it, and is then provided with an oxide film.

In another of its aspects, the present invention provides a container produced by the method just indicated, where in the pointed projections on the internal surface of the container have been worn down by a lustring treatment and an oxide film is present on the resultant smoothed surface.

The lustring treatment as such is well known in the art of finishingjewellery, and is applied to the production of light metal containers, especially beer barrels, in order to produce a highly lustred and mirror-like surface on the inside of such containers by flattening and smoothing the same before it is oxidised.

Apart from the advantage of such a smooth internal surface that it is less liable to collect dirt and is simpler to clean, the application of a lustring treatment prior to oxidation has the further advantage that an oxide film on a smooth and flattened internal surface of a container is much more lasting and durable than one applied to a surface which is full of scratches and scores. The protective layer applied by anodic or chemical oxidation will uniformly cover the smooth inside surface of the container in the form of a continuous unbroken film which exhibits no fissures or cracks. Moreover, the internal surface of a light metal barrel when treated according to the invention has a wholesome, clean, and mirror-like appearance.

The method just indicated is of especial value in beer barrels, since extensive and careful tests have disclosed that residues of beer do not tend to adhere to a lustred and oxidised surface. This affords the further advantage that less aggressive caustics or acids can be used by the brewers for cleaning the barrels so that one of the causes of corrosion will thus be automatically eliminated.

Suitable lustring processes are chemical methods, and electrolytic techniques, the former being preferred, and such processes have been described for example in British patent specifications Nos. 693,776 and 693,876, United States patent specifications Nos. 2,756,131 and 2,746,849, and in German patent specifications Nos. 661,266, 814,372 and 814,984.

It has also been found that the method according to the invention permits barrels to be repaired which already have a highly corroded internal surface and which in the past were discarded for scrap. The chemical lustring process smooths and fiattens the corroded cavities in the material and permits the internal walls to be provided with a fresh and more durable oxide film.

It has been found that a suitable lustring bath is one in which one litre of the bath contains 100 g. ammonium bifiuoride, NH F.HF, 145, cc. 54% nitric acid, HNO i.e. 36 B., 25 'g. dextrin, and 855 cc. water, the bath being preferably used at a temperature of 55 C.

The invention will now be illustrated by the following examples.

Example 1 A finished container is first internally treated with a lustring bath of the composition just indicated at a temperature of 55 C. for about 60 secs. The container is then rinsed with water, neutralised with nitric acid, and flushed out again. By anodic oxidation in a sulphuric acid bath, an oxide film of 12 to 16p. is then produced, the container being subsequently once more flushed out with water. Finally, the oxide film is consolidated by the application of steam at a temperature of about to 98 C. for a period of 60 minutes.

In the further development of the invention it was found that the corrosion-resistance of light metal containers, especially barrels, can be further improved by coating or impregnating the oxide film with a layer of varnish.

The varnish may be any stoving varnish such as alkyd resin-based varnishes, f.i., that obtainable in the trade under the name Permanax L Clear varnishes such as epicote varnishes may likewise be used. If a varnish is thus applied it is not always necessary first to consolidate the oxide film in conventional manner, for instance by treatment with steam.

The containers, especially beer barrels, treated by the process described in Example 1, may be evacuated after they have been chemically or anodically oxidised, a stoving varnish being then introduced into the barrels and pressure applied, whereupon the varnish, preferably after having been provisionally dried, is stoved on by hot air. This method will now be described in Example 2.

Example 2 A pretreated aluminum beer barrel with an inside surface that has first been smoothed and flattened by the alkyd resin-based chemical lustring process and then provided with an oxide film is evacuated until the pressure is about 10 mm. Hg. In the case of a barrel of 50 litres capacity about 200 cc. of the alkyd resin-based stoving varnish obtainable in the trade under the name Permanax L are then introduced into the evacuated barrel.

When this has been done the barrel is, for a short time, subjected to a pressure of between 1 /2 and 2 ats. gauge, for the purpose of causing the varnish to apply itself smoothly to the internal surfaces of the barrel. Warm air at a temperature between 50 and 60 C. is then introduced for 3 to minutes to pre-dry the varnish. When this treatment has been completed the barrel is placed for about 30 minutes into an oven where it is exposed to a temperature of about 120 and the varnish stoved on the surface.

The invention will now be further illustrated by reference to the accompanying diagrammatic drawing in which:

FIG. 1 illustrates, on an enlarged scale, the manner in which a pointed projection on the internal surface of a container prepared in the hitherto conventional manner causes the oxide layer to split,

FIG. 2 shows the manner in which such points are smoothed and planed down before the internal container surface is chemically or anodically oxidised,

FIG. 3 shows, on an enlarged scale, a corroded spot in a container of the hitherto conventional kind,

FIG. 4 shows the manner in which such a corroded spot is smoothed down by a lustring treatment and thus prepared for chemical or anodic oxidation,

FIG. 5 is a highly magnified section through a portion of the internal wall of a light metal beer barrel that has been internally plated and treated according to the invention, and

FIG. 6 is a highly magnified section through a portion of the internal wall of a light metal beer barrel, provided with a coating of lacquer after having been lustred and oxidised.

In the drawings equivalent parts are indicated by the same reference numerals. FIG. 1 illustrates how a pointed projection 2 in the metal surface, when provided with an oxide film 1, tends to become exposed by a fissure 3 forming in the oxide film 1 and thus to become susceptible to corrosive attack by the contents of the container.

FIG. 2 shows the nature of a metal surface comprising sharp points 2 before it has been treated in the manner proposed by the invention. After having been chemically or anodically lustred these points 2 are flattened down so that a surface conformation is produced as indicated by 4.

FIG. 3 shows a point of corrosive attack in containers or beer barrels of the kind hitherto used, that is to say in containers or barrels with an internal surface that has been considerably pitted by corrosive attack. In this drawing 5 denotes the basic material consisting of an aluminum alloy, whereas 6 is a plated film of pure aluminum. The drawing clearly shows the cavities and corroded spots 7.

FIG. 4 shows the result of a treatment according to the invention applied to a corroded barrel of the kind having an internal surface such as that shown in FIG. 3. The cavities and pitted spots 7 in FIG. 3 have disappeared and the surface has been largely flattened down and smoothed.

FIG. 5 shows the base material 5 provided with a plated layer 6 covered by an oxide film 2. In the manner that has been described this oxide film is provided with a lacquer coating 8.

Finally, FIG. 6 is a section through the internal wall of a barrel, similar to FIG. 5, but Without an internal plating 6. This illustrates the manner in which the points on the inside barrel surface are flattened and smoothed by the lustring treatment prior to oxidation. In this example a coating of lacquer 8 is likewise shown applied over the oxide film 2.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

'1. A method of producing a corrosionresistant internal surface on a container made of light metal, comprising the steps of subjecting the internal surface of the container to a lustring treatment, providing an oxide film on the resulting smooth surface, and coating said film with a layer of varnish.

2. A beer barrel having an inner wall consisting essentially of a light metal selected from the group consisting of aluminum and aluminum alloys and having a smooth lustred free surface; an oxide film formed on and cover- ;ing said surface; and a varnish coating covering said oxide 3. A beer barrel made of light metal and filled with beer said barrel including an inner wall having a continuous surface layer in contact with said beer, said surface layer consisting essentially of a varnish-covered oxide coating formed on a lustred aluminum layer.

References Cited in the file of this patent UNITED STATES PATENTS 2,196,018 Korpiun Apr. 2, 1940 2,671,996 Schneider Mar. 16, 1954 FOREIGN PATENTS 9,753 Great Britain Oct. 9, 1913 of 1913 693,776 Great Britain July 8, 1953 

2. A BEER BARREL HAVING AN INNER WALL CONSISTING ESSENTIALLY OF A LIGHT METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM ALLOYS AND HAVING A SMOOTH LUSTRED FREE SURFACE; AN OXIDE FILM FORMED ON AND COVERING SAID SURFACE; AND A VARNISH COATING COVERING SAID OXIDE FILM. 